The declaration of public utility authorizing the rebuilding with double track of the Rothau-Saales section is taken on March 23, 1921 with concession with the A.L. The part Dié Saales-Saint is already conceded, since March 28, 1920, with the Company de l'EST. The law of June 22, 1923 specifies that the future exploitation will be entrusted by it to the A.L with the carryforward of the limit of the two networks with Saint Dié. Work of doubling of the ways between Rothau and Molsheim begins in May 1925 and the building site, entrusted to the company Joinet from Thaon les Vosges, progresses of West in East. The station of Schirmeck is increased and equipped with a deposit of machines. At the beginning of 1927, the platform is widened until Gresswiller, then Mutzig. A jump of sheep is arranged between this station and Molsheim where the double track coming from Strasbourg is completed; indeed, the Dachstein-Molsheim section is still with single track in 1927. Thus, on the new line from Mutzig Saint-Dié, circulation will be carried out on the left as on the other French networks. The first Vosgean opening of the post-war period is under operation with the winter service 1930-31.